Understanding the complex interaction between groundwater and surface water (SW-GW) is essential for effective management of water resources. One of the most important results of this interaction is formation of freshwater lenses. Terrestrial Freshwater Water Lenses (TFWLs) are of great importance in areas with regionally-extensive brackish to saline groundwater. When recharge occurs through rainfall or river or lake, a TFWL will form, with freshwater floating above denser, saline groundwater derived from the sea or underlying aquifer. TFWLs in arid and semi-arid regions are critically important for the health of threatened ecosystems, and provide other positive functions within otherwise semi-arid and arid riparian settings. TFWLs have been described as some of the most vulnerable aquifer systems in the world, which can be attributed to various factors, such as episodic droughts, excessive pumping and river flow manipulation.
This study aims to quantify the potential impacts of river morphology on formation of TFWLs at an arid setup with saline floodplain aquifer. A series of river morphology scenarios are modelled based on river bank slope ranging from 90° to 15° from horizontal and meandering ranging from no-meandering to 3 bends for both gaining and losing rivers. A 400m by 800m, 10 m thick aquifer is modelled as homogeneous and isotropic with an impermeable base and constant head on the right and left hand boundary conditions and variable heads at the river nodes. To represent the arid/semi-arid regions, constant values of 200 mm/year and 1000 mm/year were assigned for rainfall and ET, respectively. The soil parameters were based on sandy-loam from the soil types Carsel and Parrish.
The problem to be modelled involved both surface and porous media domains, therefore the fully–coupled, surface–subsurface flow model HydroGeoSphere is used. HydroGeoSphere simultaneously solves the diffusion-wave approximation of the Saint Venant equation for surface water flow, and the Richards' equation governing 3D unsaturated/saturated subsurface flow with a physical coupling between the two domains.
The results of this research demonstrate the significant impacts of the examined drivers on dynamics of TFWLs at saline arid setup. Increase in river bank slope, 90° to 15° from horizontal, may lead to larger TFWLs. Moreover, meandering pattern show significant impact on FWLs distribution at the floodplain aquifer. Overall, this study is a step forward towards better understanding of dynamics of TFWLs as one of the most valuable and vulnerable water resources.